The present invention relates to a method and circuit arrangement for determining the partial pressure and the concentration of a gas, termed a measuring gas, which is mixed with at least one additional gas according to the optical absorption method.
In such methods, defined wavelength ranges are alternatingly filtered out of a beam of light of predetermined intensity by means of filters and when the light penetrates a gas to be measured, the intensity of light in a first wavelength range and/or ranges is reduced while that of light in a second wavelength range and/or ranges is not reduced. The intensity of the light radiation is measured with a radiation sensitive detector and fluctuations in output intensity from the light source, variations in the optical transmission and reflection parameters of the beam path, and other interfering values are substantially eliminated from the measurement result by way of comparison of two successive signals having different spectral distributions.
In procedures for optimizing separation nozzle systems, separating experiments operating at low inlet pressures, under small cut and employing low UF.sub.6 concentrations, i.e. very low UF.sub.6 partial pressures, are achieving increasing significance.
The cut .theta..sub.u of a separation nozzle system which splits an UF.sub.6 stream L into an inner partial stream Li and an outer partial stream Lo is L.sub.i /L.
Moreover, it has been found that, for safety reasons, continuous control of the effectiveness of UF.sub.6 low temperature separation is absolutely necessary in the case of separators operating with UF.sub.6 partial pressures below 10.sup.-4 Torr. Additionally, it is desirable, in order to accurately determine the costs for separator systems, to effect continuous measurements of the HF content of the gas being processed.
The known nonselective measuring methods, however, are incapable of providing a precise UF.sub.6 concentration determination at extremely low UF.sub.6 partial pressures because the resulting measuring signal value is determined almost exclusively by the additional gas, which is hydrogen or helium, and which is present in excess amounts. Moreover, the measured value is falsified by gaseous impurity components, e.g. hydrogen fluoride and other compounds contained in commercial UF.sub.6, which are particularly noticeable at low partial pressures. For these reasons, it is necessary to use selective measuring systems which permit separate measurements of the UF.sub.6 partial pressure and of the partial pressures of the impurity components.
In a known selective process, which is constituted by the photometer process based on the bifrequency principle, the sample contained in a cuvette system is illuminated with light which alternates between two different, but closely adjacent, wavelength ranges, one of the wavelength ranges coinciding with an absorption band of the gas component to be examined and the other wavelength range lying outside of the absorption range so that light therein is not weakened, or attenuated, by the gas. The two wavelength ranges are extracted out of the spectrum of the radiation source by means of gas filters or solid state interference filters. Pulses of light, which alternate between the two wavelength ranges, pass through the cuvette system and are detected by a detector. By comparing every two successive signals produced by light of respectively different wavelengths, fluctuations in the intensity of the light source output or variations in the optical transmission and reflection properties of the beam path are substantially eliminated, as are fluctuations in the sensitivity and the zero point of the detector and variations in background radiation, since they have almost the same effect on successive radiation pulses.
The drawbacks of this process are in particular that for some measuring problems, e.g. in connection with UF.sub.6 analysis, it is necessary to effect complicated dry gas rinsing of optical path outside of the analyzer chamber in order to eliminate e.g. the H.sub.2 O bands, since, on the one hand, the H.sub.2 O spectrum does not have the requisite gap in the region of the .nu..sub.3 band of UF.sub.6 and, on the other hand, the light path through air associated with a bifrequency grid analyzer is, in principle, relatively long.
Moreover, the measuring time of the grid analyzer is determined by the time required to switch between the two wavelength ranges required by the bifrequency principle. Due to the high accuracy with which the wavelengths must be selected, the switching frequency is about 10.sup.-2 Hz, so that the measuring times can be no less than about 100 seconds.
In contradistinction to the grid analyzers, quasi-continuous measurements are possible in principle within less than 1 second and over light paths which extend only a short distance through air, if the wavelength selection is effected by means of solid state filters disposed on a filter wheel which rotates, or a pendulum disc which swings, at a comparatively high frequency.
Such measuring problems can also be solved by use of known spectral analyzers with negative gas filtration, in which case gas filters and reference filter cells disposed on a rotating circular disc are moved alternatingly through the beam path.
The drawback of these methods is that they require high chopper frequencies which result in a time overlap between successive signals, producing measuring errors or even making useful measurements impossible.
In known infrared analyzers it has been attempted to avoid this problem by employing a double chopper system. In this case, the very low frequency of the filter chopper, which is lower than the reciprocal of the relaxation period of the thermal detector is used to switch between the two wavelength ranges while additionally the second chopper effects a high frequency interruption of the radiation. However, this arrangement requires a large amount of mechanical components to synchronize the two choppers. Additionally, due to wear of the mechanical components, there will occur temporary changes in synchronization so that long-term stability, which is required for many practical problems, cannot be attained. Furthermore, the signals produced during the switching period of the lower frequency chopper cannot be used for evaluation, i.e. the information furnished by the optical portion of the device is utilized only in part.